A paste such as a resin type paste or a calcined type paste and a coating material such as an electromagnetic wave shielding coating material which use a metal filler such as a silver powder or a silver-coated copper powder are frequently used in the formation of a wiring layer, an electrode, and the like in an electronic device. An electrically conductive film to be a wiring layer or an electrode is formed by applying or printing a metal filler paste of a silver powder or a silver-coated copper powder on various kinds of substrates and then subjecting the coated metal filler paste to heat curing or heat calcination.
For example, a resin type electrically conductive paste is composed of a metal filler, a resin, a curing agent, a solvent, and the like, and it is formed into an electrically conductive film by being printed on an electric conductor circuit pattern or a terminal and cured by heating at from 100° C. to 200° C. so as to form a wire or an electrode. In a resin type electrically conductive paste, the thermosetting resin is cured and shrunk by heat, and metal fillers are thus joined by pressure and brought into contact with one another to overlap each other, and as a result, an electrically connected current path is formed. This resin type electrically conductive paste is used in a substrate using a material susceptible to heat such as a printed wiring board since this paste is treated at a curing temperature of 200° C. or lower.
On the other hand, a calcination type electrically conductive paste is composed of a metal filler, glass, a solvent, and the like, and it is formed into an electrically conductive film by being printed on an electric conductor circuit pattern or a terminal and calcined by heating at from 600° C. to 800° C. so as to form a wire or an electrode. The calcination type electrically conductive paste is treated at a high temperature so that the metal fillers are sintered together to secure the conduction property. This calcination type electrically conductive paste cannot be used in a printed wiring board using a resin material since it is treated at a high temperature for calcination in this manner, but it is possible to realize a low resistance as the metal fillers are sintered by the high temperature treatment. Hence, a calcination type electrically conductive paste is used in an external electrode of a laminated ceramic capacitor, and the like.
On the other hand, the electromagnetic wave shield is used to prevent the generation of electromagnetic noises from an electronic device, and particularly in recent years, the housing of a personal computer or a mobile phone is made of a resin, and a method to form a thin metal film by a vapor deposition method or a sputtering method, a method to apply an electrically conductive coating material, and a method to shield electromagnetic waves by attaching an electrically conductive sheet to a required place, and the like have been thus proposed in order to secure the electrical conductivity of the housing. Among them, a method in which a metal filler is dispersed in a resin and applied and a method in which a metal filler is dispersed in a resin and processed into a sheet shape and the sheet is attached to a housing are frequently used since these methods do not require special equipment in the processing step and thus exhibit an excellent degree of freedom.
However, in such a case of dispersing a metal filler in a resin and applying the resin or processing the resin into a sheet, the dispersion state of the metal filler in the resin is not uniform, and thus a method to increase the filling factor of the metal filler is required in order to obtain an electromagnetic wave shielding efficiency. However, in this case, problems that the flexibility of the resin sheet is impaired and the like are caused as well as the weight of the sheet increases by the addition of a great amount of metal filler. Hence, in order to solve these problems, for example, in Patent Document 1, a method using a flat plate-shaped metal filler is proposed and it is described that this makes it possible to form a thin sheet exhibiting an excellent electromagnetic wave shielding effect and also favorable flexibility.
Here, in order to fabricate a flat plate-shaped copper powder, for example, Patent Document 2 discloses a method for obtaining a flaky copper powder suitable for a filler of an electrically conductive paste. Specifically, a spherical copper powder having an average particle diameter of from 0.5 to 10 μm as a raw material is mechanically processed into a flat plate shape by mechanical energy of a medium filled in the mill by using a ball mill or a vibrating mill.
In addition, for example, Patent Document 3 discloses a copper powder for an electrically conductive paste, specifically, a discoid copper powder capable of obtaining high performance as a copper paste for through holes and external electrodes and a technique relating to a production method thereof. Specifically, a granular atomized copper powder is put into a medium stirring mill, a steel ball having a diameter of ⅛ to ¼ inch is used as a grinding medium, a fatty acid is added to the copper powder at from 0.5 to 1% by weight, and the granular atomized copper powder is ground in the air or an inert atmosphere to be processed into a flat plate shape.
On the other hand, a silver powder is frequently used as the metal filler to be used in these electrically conductive pastes and electromagnetic wave shields, but there is a tendency to use a silver-coated copper powder in which the amount of silver used is decreased by coating silver on the surface of a copper powder which is more inexpensive than a silver powder due to the cost saving trend.
As a method to coat the surface of a copper powder with silver, there are a method to coat the copper surface with silver by a substitution reaction and a method to coat the copper surface with silver in an electroless plating solution containing a reducing agent.
In the method to coat the copper surface with silver by a substitution reaction, a silver film is formed on the copper surface as the silver ion is reduced by the electrons generated when copper dissolves in the solution. For example, Patent Document 4 discloses a production method in which a silver film is formed on the copper surface by the substitution reaction between copper and the silver ion as a copper powder is put into a solution in which a silver ion is present. However, in this method by a substitution reaction, there is a problem that the amount of silver coated cannot be controlled since the dissolution of copper does not proceed any more when a silver film is formed on the copper surface.
In order to solve this problem, there is a method to coat silver by using an electroless plating solution containing a reducing agent. For example, Patent Document 5 proposes a method for producing a copper powder coated with silver by the reaction between a copper powder and silver nitrate in a solution in which a reducing agent is dissolved.
Meanwhile, as the copper powder, an electrolytic copper powder precipitated in a dendritic shape called a dendritic shape is known and it is characterized by a large surface area due to the dendritic shape thereof. By having a dendritic shape as described above, there is an advantage that the amount of an electrically conductive filler in an electrically conductive paste and the like can be decreased since the branches of the dendrite overlap each other, conduction is likely to occur, and the number of contact points between the particles is greater as compared to that of spherical particles in the case of using this in an electrically conductive film and the like. For example, Patent Documents 6 and 7 propose a silver-coated copper powder in which the surface of a copper powder having a dendritic shape is coated with silver.
Specifically, Patent Documents 6 and 7 disclose dendrites characterized by long branches branched from the main axis as one that is further grown in a dendritic shape, and it is described that the silver-coated copper powder exhibits improved conduction property as the contact points between the particles are more than those between the conventional dendrites and it can enhance the electrical conductivity when being used in an electrically conductive paste and the like even though the amount of the electrically conductive powder is decreased.
On the other hand, when the metal filler has a dendritically developed shape in the case of utilizing a dendritic copper powder as a metal filler of an electrically conductive paste, a resin for electromagnetic wave shielding, and the like, the electrolytic copper powders are intertwined with one another more than the required extent, and it is thus pointed out in Patent Document 8 that it is extremely difficult to handle the electrolytic copper powder and the productivity decreases as the aggregation thereof is likely to occur and the fluidity thereof decreases. Incidentally, it is described in Patent Document 8 that it is possible to improve the strength of the electrolytic copper powder itself, to make it difficult for the dendrite to break, and to mold the electrolytic copper powder to have a high strength by adding a tungstate salt to an aqueous solution of copper sulfate that is an electrolytic solution for precipitating the electrolytic copper powder in order to increase the strength of the electrolytic copper powder itself.
In this manner, it is not easy to use a dendritic copper powder as a metal filler of an electrically conductive paste and the like and the dendritic copper powder is also a cause of poor improvement in the electrical conductivity of the paste.
In order to secure the electrical conductivity, a dendritic shape having a three-dimensional shape is more likely to secure the contact points than a granular shape and is expected to secure high electrical conductivity as an electrically conductive paste or an electromagnetic wave shield. However, a silver-coated copper powder having a conventional dendritic shape is a dendrite characterized by a long branch branched from the main axis and has an elongated branch shape so that it has a simple structure from the viewpoint of securing the contact point and does not have an ideal shape as a shape to effectively secure the contact point by using a smaller amount of silver-coated copper powder.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-258490    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2005-200734    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2002-15622    Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2000-248303    Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2006-161081    Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2013-89576    Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2013-100592    Patent Document 8: Japanese Unexamined Patent Application, Publication No. 2011-58027